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Message-Id: <20200219210103.32400-25-willy@infradead.org>
Date: Wed, 19 Feb 2020 13:01:03 -0800
From: Matthew Wilcox <willy@...radead.org>
To: linux-fsdevel@...r.kernel.org
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
linux-btrfs@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
cluster-devel@...hat.com, ocfs2-devel@....oracle.com,
linux-xfs@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
Michal Hocko <mhocko@...e.com>
Subject: [PATCH v7 24/24] mm: Use memalloc_nofs_save in readahead path
From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
Ensure that memory allocations in the readahead path do not attempt to
reclaim file-backed pages, which could lead to a deadlock. It is
possible, though unlikely this is the root cause of a problem observed
by Cong Wang.
Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
Reported-by: Cong Wang <xiyou.wangcong@...il.com>
Suggested-by: Michal Hocko <mhocko@...e.com>
---
mm/readahead.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/mm/readahead.c b/mm/readahead.c
index bbe7208fcc2d..9fb5f77dcf69 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -22,6 +22,7 @@
#include <linux/mm_inline.h>
#include <linux/blk-cgroup.h>
#include <linux/fadvise.h>
+#include <linux/sched/mm.h>
#include "internal.h"
@@ -186,6 +187,18 @@ void page_cache_readahead_unbounded(struct address_space *mapping,
};
unsigned long i;
+ /*
+ * Partway through the readahead operation, we will have added
+ * locked pages to the page cache, but will not yet have submitted
+ * them for I/O. Adding another page may need to allocate memory,
+ * which can trigger memory reclaim. Telling the VM we're in
+ * the middle of a filesystem operation will cause it to not
+ * touch file-backed pages, preventing a deadlock. Most (all?)
+ * filesystems already specify __GFP_NOFS in their mapping's
+ * gfp_mask, but let's be explicit here.
+ */
+ unsigned int nofs = memalloc_nofs_save();
+
/*
* Preallocate as many pages as we will need.
*/
@@ -230,6 +243,7 @@ void page_cache_readahead_unbounded(struct address_space *mapping,
* will then handle the error.
*/
read_pages(&rac, &page_pool);
+ memalloc_nofs_restore(nofs);
}
EXPORT_SYMBOL_GPL(page_cache_readahead_unbounded);
--
2.25.0
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